The electrothermoconvective instability of a low conducting liquid in modulated electric field of a plane horizontal capacitor is investigated in the electrohydrodynamic (EHD) approximation for arbitrary field modulation frequencies and various shapes (harmonic and triangular). The charge formation is produced either due to dielectrophoresis or due to the nonhomogeneous conductivity. The Floquet theory is used to find the convection thresholds. Stability maps of electrical Rayleigh number versus inverse frequency are given. Depending on the amplitude and frequency of modulation, the electric field can destabilize the equilibrium of the liquid. The stability of the liquid layer is also investigated with and without the effect of buoyancy.
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